The field of the proposed invention relates to high intensity blending apparatus and processes, particularly for blending operations designed to cause additive materials to become affixed to the surface of base particles. More particularly, the proposed invention relates to an improved method for producing surface modifications to electrophotographic and related toner particles.
High speed blending of dry, dispersed, or slurried particles is a common operation in the preparation of many industrial products. Examples of products commonly made using such high-speed blending operations include, without limitation, paint and colorant dispersions, pigments, varnishes, inks, pharmaceuticals, cosmetics, adhesives, food, food colorants, flavorings, beverages, rubber, and many plastic products. In some industrial operations, the impacts created during such high-speed blending are used both to uniformly mix the blend media and, additionally, to cause attachment of additive chemicals to the surface of particles (including resin molecules or conglomerates of resins and particles) in order to impart additional chemical, mechanical, and/or electrostatic properties. Such attachment between particles is typically caused by both mechanical impaction and electrostatic bonding between additives and particles as a result of the extreme pressures created by particle/additive impacts within the blender device. Among the products wherein attachments between particles and/or resins and additive particles are important during at least one stage of manufacture are paint dispersions, inks, pigments, rubber, and certain plastics.
A typical blending machine and blending tool of the prior art is exemplified in FIGS. 1 and 2. FIG. 1 is a schematic elevational view of a blending machine 2. Blending machine 2 comprises a vessel 10 into which materials to be mixed and blended are added before or during the blending process. Housing base 12 supports the weight of vessel 10 and its contents. Motor 13 is located within housing base 12 such that its drive shaft 14 extends vertically through an aperture in housing 12. Shaft 14 also extends into vessel 10 though sealed aperture 15 located at the bottom of vessel 10. Shaft 14 is fitted with a locking fixture 17 at its end, and blending tool 16 is rigidly attached to shaft 14 by locking fixture 17. Before blending is commenced, lid 18 is lowered and fastened onto vessel 10 to prevent spillage. For high intensity blending, the speed of the rotating tool at its outside edge generally exceeds 50 ft./second. The higher the speed, the more intense, and tool speeds in excess of 90 ft./second, or 100 ft./second are common.
Turning now to FIG. 2, a perspective view of blending tool 16 of the prior art is shown. Center shank 20 has a central fixture 17A for engagement by locking fixture 17 (shown in FIG. 1). In the example shown, the central fixture 17A is a simple notched hole for receiving a male fixture 17 (from FIG. 1) having the same dimensions. Arrow 21 shows the direction in which tool 16 rotates upon shaft 14. Vertical surfaces 19A and 19B are fixed to the end of center shank 20 in order to increase the surface area of the tool at its point of greatest velocity. This increases the tool's “intensity”, or number of collisions per unit of time. In addition to the surface area of the tool's face, the intensity of a tool is influenced by tool speed and the shape of the tool. The importance of the shape of the tool will be discussed below. Vertical surfaces 19A and 19B combined with the leading edge of center shank 20 are the surfaces of tool 16 that collide with particles mixed within vessel 10 (shown in FIG. 1). The area through which these surfaces 19 and leading edge of center shank 20 sweep during rotation of tool 16 can be thought of as the working profile of the tool. In other words, the “profile” of a tool equals the 2-dimensional area outlined by collision surfaces of the tool as it sweeps through a plane that includes the rotational axis of shaft 14. In FIG. 2, the space or zone immediately behind rotating tool 16 is labeled 22.
Various shapes and thicknesses of blending tools and collision surfaces are possible. Various configurations are shown in the brochures and catalogues offered by manufacturer's of high-speed blending equipment such as Henschel, Littleford Day Inc., and other vendors. The tool shown in FIG. 2 is based upon a tool for high intensity blending produced by Littleford Day, Inc. Among the reasons for different configurations of blending tools are (i) different viscosities often require differently shaped tools to efficiently utilize the power and torque of the blending motor; and (ii) different blending applications require different intensities of blending. For instance, some food processing applications may require a very fine distribution of small solid particles such as colorants and flavorings within a liquid medium. Similarly, the processing of snow cones requires rapid and very high intensity blending designed to shatter ice cubes into small particles which are then mixed within the blender with flavored syrups to form a slurry.
Most high-speed blending tools of the prior art do not have raised vertical elements such as surfaces 19 shown in FIG. 2. Instead, a typical blending tool has a collision surface formed simply by the leading edge of its central shank 20. In many tools, the leading edge is rounded or arcurately shaped in order to avoid a “snow plow” effect wherein particles become caked upon a flat leading face much as snow is compressed and forms piles in front of a snow plow. The tool shown in FIG. 2 attempts to avoid this snow plow effect on raised collision surfaces 19 by slanting the forward face of surfaces 19 at an acute angle, thereby causing particles to either bounce upward from the tool or be swept by friction upward along the face of the tool until carried over its top and into the lee of the tool. However, a problem with the tool shown in FIG. 2 and with other tools in the prior art is that an enlarged collision surface tends to create vortices in the wake of the tool as well as to decrease the overall density of particles in the zone 22 behind the tool. The degree of such density variations depends primarily upon the speed of the tool through the particle mixture as well as the height, width, and depth of the collision surface 19.
Because of the above snow plow, vortex, and density limitations, conventional tools such as shown in FIG. 2 are limited both in height and in the width of any enlarged collision surface. Indeed, it is believed that in tools of the prior art that have elements raised above center shank 20, the height (defined below as the y-axis dimension) of such vertically raised elements is less than the depth (defined below as the z-axis dimension) of center shank 20 in its region proximate to the attachment point of the enlarged element. It is also believed that the width (defined below as the x-axis dimension) of any vertically raised element of a conventional tool has not exceeded the height, or y-axis, of center shank 20 in the region of center shank 20 proximate to where the raised element is attached. Lastly, it is believed that in high-speed blending tools of the prior art that have raised elements, the z-axis dimension, or depth, of the raised element greatly exceeds its width, or x-axis, dimension. For clarification, the height, or y-axis, dimension of a blending tool and its elements shall mean the dimension of the tool or element in the plane that contains shaft 14 around which the tool rotates. The depth, or z-axis, of the tool and its elements shall mean the dimension perpendicular both to the axis of the tool's center shank and to the y-axis. The x-axis of the tool and its elements shall be measured in the direction of the axis of the tool's center shank. For center shank 20 itself, the x-axis dimension is a measure of its length. For any raised collision surface, the x-axis is a measure of its width.
Another characteristic of blending tools of the prior art is derived from the above limitations upon the height of the collision surface. Specifically, as explained above, conventional tools are thin in height and, if a vertical surface such as 19 is present, such vertical surface is also has a thin x-axis profile. Such thinness is required in order to avoid excessive vortices and low density regions in the lee of the tool. The trailing edges of conventional tools are sometimes rounded or arcurately shaped. However, because of the “thinness” of the tool in the y-axis, it is not necessary and it is not known to arcurately shape the leading or trailing surfaces of the tool except in the region proximate to the leading and/or trailing edge.
As noted above, different mixture formulations or products often specify different collision surface shapes and dimensions in order to optimize blend efficiency, blend time, and power consumption. For instance, if a fast blend process time is desired, then the blend tool can be rotated faster or a tool with a larger collision surface can be selected in order to increase the number of particle collisions per unit of time, or blending intensity. However, for any given viscosity, the power and configuration of the blending motor effectively limits the speed of the tool and the size of a collision surface such as surface 19.
When the same blending vessel is used for different formulations or products requiring different tools, then procedures for changing a conventional blending tool require the following steps (described in relation to FIG. 1) (A) lid 17 is unfastened and opened from the top of vessel 10; (B) vessel 10 and tool 16 need to be at least partially cleaned by vacuum and by wiping, especially in the region where blending tool 16 is secured to shaft 14; (C) locking fixture 17 is loosened to allow unfastening of tool 16 from shaft 14; (D) blending tool 16 is detached from the locking fixture 17; (D) blending tool 16 is lifted from vessel 10 with care not to bump or scratch the sides of vessel 10; (F) removed tool 16 is thoroughly cleaned before further handling and/or storage; and (G) the preceding tasks (except cleaning) are repeated in reverse order for attachment of a different blending tool 16. For large blender vessels that are common in many if not most industrial applications, the weight of blending tool 16 requires a crane or hoist during unfastening, lifting, positioning of the replacement tool, and refastening. A human operator inside vessel 10 typically needs to help maneuver the crane or hoist during this process, and the combination of positioning a large tool while simultaneously attempting to fasten it onto shaft 14 can place the human operator in an awkward position. Even for smaller blenders, replacement of the tool requires fairly careful cleaning of shaft 14 and tool 16 and often requires an awkward manipulation while simultaneously positioning and fastening replacement tool 16.
In addition to changing a blending tool to accommodate the requirements of different formulations or products, blending tools may require changing when excessively worn. Many industrial applications require blending of abrasive particles such as pigments, colorants (including carbon black), and electrophotographic toners. The above procedures for changing a tool must be used whenever a worn tool requires replacement.
The relevance of the above description of blending tool 16 to the manufacture of electrophotographic, electrostatic or similar toners is demonstrated by the following description of a typical toner manufacturing process. A typical polymer based toner is produced by melt-mixing the heated polymer resin with a pigment in an extruder, such as a Weiner Pfleider ZSK-53, whereby the pigment is dispersed in the polymer. After the resin has been extruded, the resin mixture is reduced in size by any suitable method including those known in the art. Such reduction is aided by the brittleness of most toners which causes the resin to fracture when impacted. This allows rapid particle size reduction in pulverizers or attritors such as media mills, jet mills, hammer mills, or similar devices. An example of a suitable hammer mill is an Alpine RTM Hammer Mill. Such a hammer mill is capable of reducing typical toner particles to a size of about 10 microns to about 30 microns. For color toners, toner particle sizes may average within an even smaller range of 4-10 microns.
After reduction of particle size by grinding or pulverizing, a classification process sorts the particles according to size. Particles classified as too large are typically fed back into the grinder or pulverizer for further reduction. Particles within the accepted range are passed onto the next toner manufacturing process.
After classification, the next typical process is a high speed blending process wherein surface additive particles are mixed with the classified toner particles within a high speed blender. These additives include but are not limited to stabilizers, waxes, flow agents, other toners and charge control additives. Specific additives suitable for use in toners include fumed silica, silicon derivatives such as Aerosil.RTM. R972, available from Degussa, Inc., ferric oxide, hydroxy terminated polyethylenes such as Unilin RTM., polyolefin waxes, which preferably are low molecular weight materials, including those with a molecular weight of from about 1,000 to about 20,000, and including polyethylenes and polypropylenes, polymethylmethacrylate, zinc stearate, chromium oxide, aluminum oxide, titanium oxide, stearic acid, and polyvinylidene fluorides such as Kynar. In aggregate these additives are typically present in amounts of from about 0.1 to about 1 percent by weight of toner particles. More specifically, zinc stearate shall preferably be present in an amount of from about 0.4 to about 0.6 weight percent. Similar amounts of Aerosi.RTM. is preferred. For proper attachment and functionality, typical additive particle sizes range from 5 nanometers to 50 nanometers. Some newer toners require a greater number of additive particles than prior toners as well as a greater proportion of additives in the 25-50 nanometer range. When combined with smaller toner particle sizes required by color toners, the increased size and coverage of additive particles for some color toners creates increased need for high intensity blending.
The above additives are typically added to the pulverized toner particles in a high speed blender such as a Henschel Blender FM-10, 75 or 600 blender. The high intensity blending serves to break additive agglomerates into the appropriate nanometer size, evenly distribute the smallest possible additive particles within the toner batch, and attach the smaller additive particles to toner particles. Each of these processes occurs concurrently within the blender. Additive particles become attached to the surface of the pulverized toner particles during collisions between particles and between particles and the blending tool as it rotates. It is believed that such attachment between toner particles and surface additives occurs due to both mechanical impaction and electrostatic attractions. The amount of such attachments is proportional to the intensity level of blending which, in turn, is a function of both the speed and shape (particularly size) of the blending tool. The amount of time used for the blending process plus the intensity determines how much energy is applied during the blending process. For this purpose, “intensity” means the number of particle collisions per unit of time. For an efficient blending tool that avoids snow plowing and excessive vortices and low density regions, “intensity” can be effectively measured by reference to the power per unit mass (typically expressed as W/lb) of the blending motor driving the blending tool. Using a standard Henschel Blender tool to manufacture conventional toners, the blending times typically range from one (1) minute to twenty (20) minutes per typical batch of 60-1000 kilograms. For certain more recent toners such as toners for Xerox Docucenter 265 and related multifunctional printers, blending speed and times are increased in order to assure that multiple layers of surface additives become attached to the toner particles. Additionally, for those toners that require a greater proportion of additive particles in excess of 25 nanometers, more blending speed and time is required to force the larger additives into the base resin particles.
The process of manufacturing toners is completed by a screening process to remove toner agglomerates and other large debris. Such screening operation may typically be performed using a Sweco Turbo screen set to 37 to 105 micron openings.
The above description of a process to manufacture an electrophotographic toner may be varied depending upon the requirements of particular toners. In particular, for full process color printing, colorants typically comprise yellow, cyan, magenta, and black colorants added to separate dispersions for each color toner. Colored toner typically comprises much smaller particle size than black toner, in the order of 4-10 microns. The smaller particle size makes the manufacturing of the toner more difficult with regard to material handling, classification and blending.
The above general description of a process for making electrophotographic toners is well known in the art. More information concerning methods and apparatus for manufacture of toner are available in the following U.S. patents, and each of the disclosures of which are incorporated herein: U.S. Pat. No. 4,338,380 issued to Erickson, et al; U.S. Pat. No. 4,298,672 issued to Chin; U.S. Pat. No. 3,944,493 issued to Jadwin; U.S. Pat. No. 4,007,293 issued to Mincer, et al; U.S. Pat. No. 4,054,465 issued to Ziobrowski; U.S. Pat. No. 4,079,014 issued to Burness, et al; U.S. Pat. No. 4,394,430 issued to Jadwin, et al; U.S. Pat. No. 4,433,040 issued to Niimura, et al; U.S. Pat. No. 4,845,003 issued to Kiriu, et al; U.S. Pat. No. 4,894,308 issued to Mahabadi et al.; U.S. Pat. No. 4,937,157 issued to Haack, et al; U.S. Pat. No. 4,937,439 issued to Chang et al.; U.S. Pat. No. 5,370,962 issued to Anderson, et al; U.S. Pat. No. 5,624,079 issued to Higuchi et al.; U.S. Pat. No. 5,716,751 issued to Bertrand et al.; U.S. Pat. No. 5,763,132 issued to Ott et al.; U.S. Pat. No. 5,874,034 issued to Proper et al.; and U.S. Pat. No. 5,998,079 issued to Tompson et al.
As described above, the process of blending plays an increasingly important role in the manufacture of electrophotographic and similar toners. It would be advantageous if an apparatus and method were found to accelerate the blending process and to thereby diminish the time and cost required for blending. Similarly, since different formulations and products often require different blending speed and intensities, it would be advantageous if an apparatus and method were found to allow a single blending tool to be reconfigured in situ for various blending intensities rather than requiring cleaning, removal, and replacement of the entire blending tool for each required change in intensity.